1. Field of the Invention
This invention relates to an electric power steering (EPS) apparatus.
2. Description of the Related Art
EPS apparatuses using a motor as a drive source have been used as power steering units for vehicles. A variety of suggestions have been made to realize better steering feeling in the EPS by using high controllability thereof.
For example, Japanese Patent Application Publication No. 2006-131191 (JP-A-2006-131191) discloses a configuration in which characteristics of each compensation control are changed based on a ratio (assist gradient) of a variation of a basic assist component to a variation of a steering torque. Thus, the EPS usually executes an operation of providing an assist force corresponding to a steering torque detected based on a torsion angle of a torsion bar provided at the steering shaft, and the variation of the assist gradient is equivalent to the variation of a spring constant of the torsion bar. This configuration makes it possible to realize good steering feeling by optimizing the characteristics of each compensation control according to the variation of such basic steering characteristics.
Further, the EPS features a degree of freedom in layout selection that is higher and an energy consumption amount that is lower than those of hydraulic power steering apparatuses. For this reason, the EPS systems have been finding ever growing application to vehicles with a small load on a front shaft of the vehicle, such as small automobiles and the so-called mid-ship vehicles. However, although such vehicles feature a small motor torque, a problem associated therewith is that a response feeling during steering, that is, a steering reaction force, is often insufficient. Accordingly, a configuration has been disclosed (see, for example, Japanese Patent Application Publication No. 2002-293256 (JP-A-2002-293256)) in which a steering reaction force component in the direction opposite that of the steering operation is superimposed on the basic assist component. Furthermore, better steering feeling can be also realized by introducing a feedback control when the steering reaction force component is computed.
However, a problem arises when the above-described two conventional configurations are used together. Thus, the execution of the torque feedback control changes an apparent spring constant of the torsion bar. As a result, it is impossible to optimize each compensation control by simply combining the two configurations and, therefore, the control stability and steering feeling are degraded.